/*
 Copyright (c) 2008, Adobe Systems Incorporated
 All rights reserved.

 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions are
 met:

 * Redistributions of source code must retain the above copyright notice,
 this list of conditions and the following disclaimer.

 * Redistributions in binary form must reproduce the above copyright
 notice, this list of conditions and the following disclaimer in the
 documentation and/or other materials provided with the distribution.

 * Neither the name of Adobe Systems Incorporated nor the names of its
 contributors may be used to endorse or promote products derived from
 this software without specific prior written permission.

 THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
 IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
 THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
 CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

package com.adobe.serialization.json {

public class JSONTokenizer {

    /**
     * Flag indicating if the tokenizer should only recognize
     * standard JSON tokens.  Setting to <code>false</code> allows
     * tokens such as NaN and allows numbers to be formatted as
     * hex, etc.
     */
    private var strict:Boolean;

    /** The object that will get parsed from the JSON string */
    private var obj:Object;

    /** The JSON string to be parsed */
    private var jsonString:String;

    /** The current parsing location in the JSON string */
    private var loc:int;

    /** The current character in the JSON string during parsing */
    private var ch:String;

    /**
     * The regular expression used to make sure the string does not
     * contain invalid control characters.
     */
    private var controlCharsRegExp:RegExp = /[\x00-\x1F]/;

    /**
     * Constructs a new JSONDecoder to parse a JSON string
     * into a native object.
     *
     * @param s The JSON string to be converted
     *        into a native object
     */
    public function JSONTokenizer(s:String, strict:Boolean) {
        jsonString = s;
        this.strict = strict;
        loc = 0;

        // prime the pump by getting the first character
        nextChar();
    }

    /**
     * Gets the next token in the input sting and advances
     * the character to the next character after the token
     */
    public function getNextToken():JSONToken {
        var token:JSONToken = new JSONToken();

        // skip any whitespace / comments since the last
        // token was read
        skipIgnored();

        // examine the new character and see what we have...
        switch (ch) {
            case '{':
                token.type = JSONTokenType.LEFT_BRACE;
                token.value = '{';
                nextChar();
                break

            case '}':
                token.type = JSONTokenType.RIGHT_BRACE;
                token.value = '}';
                nextChar();
                break

            case '[':
                token.type = JSONTokenType.LEFT_BRACKET;
                token.value = '[';
                nextChar();
                break

            case ']':
                token.type = JSONTokenType.RIGHT_BRACKET;
                token.value = ']';
                nextChar();
                break

            case ',':
                token.type = JSONTokenType.COMMA;
                token.value = ',';
                nextChar();
                break

            case ':':
                token.type = JSONTokenType.COLON;
                token.value = ':';
                nextChar();
                break;

            case 't': // attempt to read true
                var possibleTrue:String = "t" + nextChar() + nextChar() + nextChar();

                if (possibleTrue == "true") {
                    token.type = JSONTokenType.TRUE;
                    token.value = true;
                    nextChar();
                }
                else {
                    parseError("Expecting 'true' but found " + possibleTrue);
                }

                break;

            case 'f': // attempt to read false
                var possibleFalse:String = "f" + nextChar() + nextChar() + nextChar() + nextChar();

                if (possibleFalse == "false") {
                    token.type = JSONTokenType.FALSE;
                    token.value = false;
                    nextChar();
                }
                else {
                    parseError("Expecting 'false' but found " + possibleFalse);
                }

                break;

            case 'n': // attempt to read null
                var possibleNull:String = "n" + nextChar() + nextChar() + nextChar();

                if (possibleNull == "null") {
                    token.type = JSONTokenType.NULL;
                    token.value = null;
                    nextChar();
                }
                else {
                    parseError("Expecting 'null' but found " + possibleNull);
                }

                break;

            case 'N': // attempt to read NaN
                var possibleNaN:String = "N" + nextChar() + nextChar();

                if (possibleNaN == "NaN") {
                    token.type = JSONTokenType.NAN;
                    token.value = NaN;
                    nextChar();
                }
                else {
                    parseError("Expecting 'NaN' but found " + possibleNaN);
                }

                break;

            case '"': // the start of a string
                token = readString();
                break;

            default:
                // see if we can read a number
                if (isDigit(ch) || ch == '-') {
                    token = readNumber();
                }
                else if (ch == '') {
                    // check for reading past the end of the string
                    return null;
                }
                else {
                    // not sure what was in the input string - it's not
                    // anything we expected
                    parseError("Unexpected " + ch + " encountered");
                }
        }

        return token;
    }

    /**
     * Attempts to read a string from the input string.  Places
     * the character location at the first character after the
     * string.  It is assumed that ch is " before this method is called.
     *
     * @return the JSONToken with the string value if a string could
     *        be read.  Throws an error otherwise.
     */
    private function readString():JSONToken {
        // Rather than examine the string character-by-character, it's
        // faster to use indexOf to try to and find the closing quote character
        // and then replace escape sequences after the fact.

        // Start at the current input stream position
        var quoteIndex:int = loc;
        do
        {
            // Find the next quote in the input stream
            quoteIndex = jsonString.indexOf("\"", quoteIndex);

            if (quoteIndex >= 0) {
                // We found the next double quote character in the string, but we need
                // to make sure it is not part of an escape sequence.

                // Keep looping backwards while the previous character is a backslash
                var backspaceCount:int = 0;
                var backspaceIndex:int = quoteIndex - 1;
                while (jsonString.charAt(backspaceIndex) == "\\") {
                    backspaceCount++;
                    backspaceIndex--;
                }

                // If we have an even number of backslashes, that means this is the ending quote
                if (backspaceCount % 2 == 0) {
                    break;
                }

                // At this point, the quote was determined to be part of an escape sequence
                // so we need to move past the quote index to look for the next one
                quoteIndex++;
            }
            else // There are no more quotes in the string and we haven't found the end yet
            {
                parseError("Unterminated string literal");
            }
        } while (true);

        // Unescape the string
        // the token for the string we'll try to read
        var token:JSONToken = new JSONToken();
        token.type = JSONTokenType.STRING;
        // Attach resulting string to the token to return it
        token.value = unescapeString(jsonString.substr(loc, quoteIndex - loc));

        // Move past the closing quote in the input string.  This updates the next
        // character in the input stream to be the character one after the closing quote
        loc = quoteIndex + 1;
        nextChar();

        return token;
    }

    /**
     * Convert all JavaScript escape characters into normal characters
     *
     * @param input The input string to convert
     * @return Original string with escape characters replaced by real characters
     */
    public function unescapeString(input:String):String {
        // Issue #104 - If the string contains any unescaped control characters, this
        // is an error in strict mode
        if (strict && controlCharsRegExp.test(input)) {
            parseError("String contains unescaped control character (0x00-0x1F)");
        }

        var result:String = "";
        var backslashIndex:int = 0;
        var nextSubstringStartPosition:int = 0;
        var len:int = input.length;
        do
        {
            // Find the next backslash in the input
            backslashIndex = input.indexOf('\\', nextSubstringStartPosition);

            if (backslashIndex >= 0) {
                result += input.substr(nextSubstringStartPosition, backslashIndex - nextSubstringStartPosition);

                // Move past the backslash and next character (all escape sequences are
                // two characters, except for \u, which will advance this further)
                nextSubstringStartPosition = backslashIndex + 2;

                // Check the next character so we know what to escape
                var afterBackslashIndex:int = backslashIndex + 1;
                var escapedChar:String = input.charAt(afterBackslashIndex);
                switch (escapedChar) {
                    // Try to list the most common expected cases first to improve performance

                    case '"':
                        result += '"';
                        break; // quotation mark
                    case '\\':
                        result += '\\';
                        break; // reverse solidus
                    case 'n':
                        result += '\n';
                        break; // newline
                    case 'r':
                        result += '\r';
                        break; // carriage return
                    case 't':
                        result += '\t';
                        break; // horizontal tab

                    // Convert a unicode escape sequence to it's character value
                    case 'u':

                        // Save the characters as a string we'll convert to an int
                        var hexValue:String = "";

                        // Make sure there are enough characters in the string leftover
                        if (nextSubstringStartPosition + 4 > len) {
                            parseError("Unexpected end of input.  Expecting 4 hex digits after \\u.");
                        }

                        // Try to find 4 hex characters
                        for (var i:int = nextSubstringStartPosition; i < nextSubstringStartPosition + 4; i++) {
                            // get the next character and determine
                            // if it's a valid hex digit or not
                            var possibleHexChar:String = input.charAt(i);
                            if (!isHexDigit(possibleHexChar)) {
                                parseError("Excepted a hex digit, but found: " + possibleHexChar);
                            }

                            // Valid hex digit, add it to the value
                            hexValue += possibleHexChar;
                        }

                        // Convert hexValue to an integer, and use that
                        // integer value to create a character to add
                        // to our string.
                        result += String.fromCharCode(parseInt(hexValue, 16));
                        // Move past the 4 hex digits that we just read
                        nextSubstringStartPosition += 4;
                        break;

                    case 'f':
                        result += '\f';
                        break; // form feed
                    case '/':
                        result += '/';
                        break; // solidus
                    case 'b':
                        result += '\b';
                        break; // bell
                    default:
                        result += '\\' + escapedChar; // Couldn't unescape the sequence, so just pass it through
                }
            }
            else {
                // No more backslashes to replace, append the rest of the string
                result += input.substr(nextSubstringStartPosition);
                break;
            }

        } while (nextSubstringStartPosition < len);

        return result;
    }

    /**
     * Attempts to read a number from the input string.  Places
     * the character location at the first character after the
     * number.
     *
     * @return The JSONToken with the number value if a number could
     *         be read.  Throws an error otherwise.
     */
    private function readNumber():JSONToken {
        // the string to accumulate the number characters
        // into that we'll convert to a number at the end
        var input:String = "";

        // check for a negative number
        if (ch == '-') {
            input += '-';
            nextChar();
        }

        // the number must start with a digit
        if (!isDigit(ch)) {
            parseError("Expecting a digit");
        }

        // 0 can only be the first digit if it
        // is followed by a decimal point
        if (ch == '0') {
            input += ch;
            nextChar();

            // make sure no other digits come after 0
            if (isDigit(ch)) {
                parseError("A digit cannot immediately follow 0");
            }
            // unless we have 0x which starts a hex number, but this
            // doesn't match JSON spec so check for not strict mode.
            else if (!strict && ch == 'x') {
                // include the x in the input
                input += ch;
                nextChar();

                // need at least one hex digit after 0x to
                // be valid
                if (isHexDigit(ch)) {
                    input += ch;
                    nextChar();
                }
                else {
                    parseError("Number in hex format require at least one hex digit after \"0x\"");
                }

                // consume all of the hex values
                while (isHexDigit(ch)) {
                    input += ch;
                    nextChar();
                }
            }
        }
        else {
            // read numbers while we can
            while (isDigit(ch)) {
                input += ch;
                nextChar();
            }
        }

        // check for a decimal value
        if (ch == '.') {
            input += '.';
            nextChar();

            // after the decimal there has to be a digit
            if (!isDigit(ch)) {
                parseError("Expecting a digit");
            }

            // read more numbers to get the decimal value
            while (isDigit(ch)) {
                input += ch;
                nextChar();
            }
        }

        // check for scientific notation
        if (ch == 'e' || ch == 'E') {
            input += "e"
            nextChar();
            // check for sign
            if (ch == '+' || ch == '-') {
                input += ch;
                nextChar();
            }

            // require at least one number for the exponent
            // in this case
            if (!isDigit(ch)) {
                parseError("Scientific notation number needs exponent value");
            }

            // read in the exponent
            while (isDigit(ch)) {
                input += ch;
                nextChar();
            }
        }

        // convert the string to a number value
        var num:Number = Number(input);

        if (isFinite(num) && !isNaN(num)) {
            // the token for the number that we've read
            var token:JSONToken = new JSONToken();
            token.type = JSONTokenType.NUMBER;
            token.value = num;
            return token;
        }
        else {
            parseError("Number " + num + " is not valid!");
        }

        return null;
    }

    /**
     * Reads the next character in the input
     * string and advances the character location.
     *
     * @return The next character in the input string, or
     *        null if we've read past the end.
     */
    private function nextChar():String {
        return ch = jsonString.charAt(loc++);
    }

    /**
     * Advances the character location past any
     * sort of white space and comments
     */
    private function skipIgnored():void {
        var originalLoc:int;

        // keep trying to skip whitespace and comments as long
        // as we keep advancing past the original location
        do
        {
            originalLoc = loc;
            skipWhite();
            skipComments();
        }
        while (originalLoc != loc);
    }

    /**
     * Skips comments in the input string, either
     * single-line or multi-line.  Advances the character
     * to the first position after the end of the comment.
     */
    private function skipComments():void {
        if (ch == '/') {
            // Advance past the first / to find out what type of comment
            nextChar();
            switch (ch) {
                case '/': // single-line comment, read through end of line

                    // Loop over the characters until we find
                    // a newline or until there's no more characters left
                    do
                    {
                        nextChar();
                    }
                    while (ch != '\n' && ch != '')

                    // move past the \n
                    nextChar();

                    break;

                case '*': // multi-line comment, read until closing */

                    // move past the opening *
                    nextChar();

                    // try to find a trailing */
                    while (true) {
                        if (ch == '*') {
                            // check to see if we have a closing /
                            nextChar();
                            if (ch == '/') {
                                // move past the end of the closing */
                                nextChar();
                                break;
                            }
                        }
                        else {
                            // move along, looking if the next character is a *
                            nextChar();
                        }

                        // when we're here we've read past the end of
                        // the string without finding a closing */, so error
                        if (ch == '') {
                            parseError("Multi-line comment not closed");
                        }
                    }

                    break;

                // Can't match a comment after a /, so it's a parsing error
                default:
                    parseError("Unexpected " + ch + " encountered (expecting '/' or '*' )");
            }
        }

    }


    /**
     * Skip any whitespace in the input string and advances
     * the character to the first character after any possible
     * whitespace.
     */
    private function skipWhite():void {
        // As long as there are spaces in the input
        // stream, advance the current location pointer
        // past them
        while (isWhiteSpace(ch)) {
            nextChar();
        }

    }

    /**
     * Determines if a character is whitespace or not.
     *
     * @return True if the character passed in is a whitespace
     *    character
     */
    private function isWhiteSpace(ch:String):Boolean {
        // Check for the whitespace defined in the spec
        if (ch == ' ' || ch == '\t' || ch == '\n' || ch == '\r') {
            return true;
        }
        // If we're not in strict mode, we also accept non-breaking space
        else if (!strict && ch.charCodeAt(0) == 160) {
            return true;
        }

        return false;
    }

    /**
     * Determines if a character is a digit [0-9].
     *
     * @return True if the character passed in is a digit
     */
    private function isDigit(ch:String):Boolean {
        return ( ch >= '0' && ch <= '9' );
    }

    /**
     * Determines if a character is a hex digit [0-9A-Fa-f].
     *
     * @return True if the character passed in is a hex digit
     */
    private function isHexDigit(ch:String):Boolean {
        return ( isDigit(ch) || ( ch >= 'A' && ch <= 'F' ) || ( ch >= 'a' && ch <= 'f' ) );
    }

    /**
     * Raises a parsing error with a specified message, tacking
     * on the error location and the original string.
     *
     * @param message The message indicating why the error occurred
     */
    public function parseError(message:String):void {
        throw new JSONParseError(message, loc, jsonString);
    }
}

}
